The invention relates to a circuit for the detection of short voltage glitches in a supply voltage, comprising a comparator with a first input and a second input and an output for indicating a detected voltage glitch.
A circuit arrangement of the type defined in the opening paragraph is known from WO 98/50859. Said circuit is an integrated part of a so-termed power-on reset circuit which is used for bringing the elements of an associated integrated microelectronic circuit to a defined output state after a supply voltage has been switched on. The conventional power-on reset circuit generally works as an integrated circuit and therefore always needs to have an adequate time space for registration of a fluctuation of the supply voltage. Therefore, very short (typically shorter than 1 xcexcs) and deep power supply glitches which are referred to as so-termed glitch pulses pass unnoticed by unbuffered operating circuits. These glitch pulses are very hard to detect, but may disturb the behavior of the circuit and may even cause malfunctioning. According to WO 98/50859 a known power-on reset circuit is extended by a glitch detector which is particularly tuned to detecting short voltage glitches so that, subsequently, the circuit can be reset reliably.
The basic principle used in said glitch detector comprises the calculated discharge of a capacitor in case of a voltage glitch in the supply voltage and the evaluation of the subsequent charging curve of the capacitor by means a comparator. The glitch detector then needs to have a reference voltage for the negative comparator input and an additional reference voltage for the gate of a transistor. Such reference voltages, however, are not available at any rate when the supply voltage is turned on, which restricts the use of the circuit. In addition, the known circuit of the glitch detector strongly depends on the threshold voltages of the transistors used.
Against this background it is an object of the present invention to provide a circuit for the detection of short voltage glitches in a supply voltage, which circuit has a wider range of application and a higher function reliability.
This object is achieved by means of a circuit having the characteristic features of claim 1. Advantageous embodiments are contained in the dependent claims.
Accordingly, the circuit comprises:
a) a comparator having a first input and a second input as well as an output for indicating a detected voltage glitch;
b) a voltage divider connected between the supply voltage and ground, from which voltage divider the first input of the comparator taps a first and the second input of the comparator taps a second voltage;
c) a first capacitor which is connected between the first input of the comparator and ground;
d) an interrupter circuit which interrupts the connection between the first input of the comparator and the voltage divider if there is a voltage glitch in the supply voltage. The two inputs of the comparator are supplied with two different voltages by the circuit described when the supply voltage is turned on, the magnitude of which different voltages can be freely adjusted by means of the taps from the voltage divider. One of the two voltages (preferably the one closer to the ground potential) is additionally imposed on the first capacitor when there is a steady voltage. In case of a voltage glitch the connection between the first input of the comparator and the voltage divider is then interrupted, so that the first input is still connected only to the first capacitor and is thus kept at the previous voltage level that is stored in the capacitor. The other input of the comparator, on the other hand, is unchanged connected to the voltage divider, so that it proportionally follows the voltage glitch. If the voltage glitch is deep enough then, there is a state of the variable potential at the second input of the comparator beyond the voltage of the invariable first input coming closer to the ground potential. Therefore, there is a reversal of polarities at the inputs of the comparator, after which a corresponding signal is generated at the comparator output. This signal then indicates the detection of a voltage glitch.
The circuit according to the invention makes it possible by varying the taps from the voltage divider to change the level of the voltage glitch to be detected and thus flexibly adjust the circuit to the respective needs. In addition, no reference voltages are needed at the comparator inputs.
According to a further embodiment of the interrupter circuit it comprises a first transistor whose source-drain path is situated in the connection line between the first input of the comparator and the voltage divider, and whose gate is controlled by a voltage which, proportionally to the voltage glitch, comes closer to the ground potential if there is a voltage glitch. The transistor which is connected in forward direction if the supply voltage is undisturbed is then cut-off and separates the first comparator input from the voltage divider, so that the input is uncoupled from the voltage glitch.
Preferably the interrupter circuit additionally comprises the following elements:
e) a second capacitor which is connected between ground and the second input of the comparator;
f) a third capacitor, one terminal of which is connected to the supply voltage;
g) a second transistor whose source is connected to the second terminal of the above-mentioned third capacitor and whose drain and gate are connected to the second input of the comparator;
h) a connection line from the source of the second transistor to the gate of the first transistor.
An interrupter circuit comprising the above components is capable of generating a cut-off voltage at the gate of the first transistor, which cut-off voltage is proportional to the voltage glitch of the supply voltage. The threshold voltage of the transistors plays a very subordinate role for the reliable functioning of the whole circuit because the transistors operate as switches and do not serve to detect a voltage level.